Cement dispersants (water reducers) can be absorbed on cement particles to inhibit the condensation of cement particles and improve the dispersion of concrete in use. There are many substances used as cement dispersant, including lignosulphonate, naphthalene sulfonate/formaldehyde condensation polymer, phenol/p-aminobenzene sulfonic acid/formaldehyde condensation polymer, melamine sulfonate/formaldehyde condensation polymer and comb-shaped polycarboxylic acid type copolymer, etc. From the perspective of the structure, the former cement dispersants mainly take sulfonyl as the absorbing group, so having the disadvantages of lack of effective side chains providing a steric hindrance, simple molecular structure, poor adjustability, limited dispersion effect on cement, high dosage and low water reduction. The comb-shaped polycarboxylic acid type copolymer, with both the main chains rich in absorbing groups providing absorption function and the side chains having good hydrophilicity providing steric hindrance function, can improve the cement's dispersion greatly. Although the comb-shaped polycarboxylic acid type copolymer makes greater improvements in respect of comonomer and copolymerization technology with the development of concrete towards high performance, it still cannot satisfy the needs of modern constructional engineering for the high-performance cement dispersant. To develop polycarboxylic acid type copolymers with new structure has become one direction to improve the performance of polycarboxylic acid type cement dispersant. The new structures, for example, star-shaped, hyperbranched and tree-shaped, not only greatly increase the number of the absorbing groups in the molecular structure of polymer, but enhance the steric hindrance provided by the hydrophilic side chains in the comb-shaped structure. Patent CN101580353 reported a hyperbranched polycarboxylate high-efficiency water reducer and its preparation method. The preparation method includes two step: Step 1, take N,N-dimethylformamide as the dissolvent, and copolymerize acrylic ester, sodium methylacryl sulfonate and allyl polyethenoxy ether sulfate to form the main chain of the polymer, wherein ACVA is used as the initiator and the carboxyls are introduced into the end of the polymer chain; Step 2, graft the hyperbranched polyamide structures to the ends of the main poly-chains by taking advantages of the amidation between the quadrol and the carboxyl, and Michael addition reaction between the amido and the double bonds of acrylic acid successively. Moreover, with the carboxyl at the end of the hyperbranched polyamide structures, the absorbing groups in the molecular structure of the polymer are greatly increased. However, this method can only be used to introduce the hyperbranched structure to both ends of polymer chains, and a high amount of organic solvent N,N-dimethylformamide is required due to complicated synthetic procedure, which results in environmental pollution. In addition, some materials and the initiator are expensive, so the cost is high. Hence, it is imperative to develop a simple, economic, high-performance hyperbranched polycarboxylic acid type copolymer cement dispersant and its preparation method.